<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <button id="btn1">按钮1</button>
  <button id="btn2">按钮2</button>
  <button onclick="demo()">按钮3</button>

  <script>
    let btn1=document.getElementById('btn1');
    let btn2=document.getElementById('btn2');

    btn1.addEventListener('click',()=>{
      alert('点击了按钮1')
    });

    btn2.onclick=()=>{
      alert('点击了按钮2')
    }

    function demo(){
      alert('点击了按钮3')
    }
  </script>
</body>
</html>